1. Field of the Invention
The present invention pertains to a sensitive magnetic field detector including a wire coil which provides a bias current to a hall effect device which senses the same incident magnetic field as the coil. The final output signal from the hall effect device is proportional to the square of the magnetic field intensity and provides improved sensitivity of the circuit for applications including detection of magnetic anomalies such as caused by cracks in fluid transmission pipelines.
2. Background
There are many applications of sensors and switching devices in which a minute change in a magnetic field indicates an event or anomaly to be detected. One such application is in detecting stress or corrosion cracks in fluid transmission pipelines and similar structures. It has, for example, been proposed to use a wire coil disposed in a magnetic field which passes through the wall of the pipeline or similar structure in which the crack or anomaly is disposed. The magnetic field is typically passed adjacent to the structure in the area to be sensed and the change in the field caused by the crack or other anomaly generates a current flow which is proportional to the number of turns in the coil, the strength of the magnetic field, and the rate of change in the intensity of the field. However, the sensitivity of a simple coil type detector may not be sufficient in detecting minute cracks in the range of 10 to 100 microinches, particularly considering circuit noise, less than ideal field flux contrast as the detector passes by the crack, flux leakage and the like. Considering these factors the limit of detectability of stress or corrosion cracks in steel pipe may be in the range of 1,000 microinches or greater.
Hall effect devices or elements are also well-known for use in detecting the presence of magnetic fields. Since a Hall device is not flux rate sensitive the detection of an anomaly or crack in a structure by passing a Hall device disposed in a magnetic field which is moving relative to the crack may be confined to rather low relative velocities. However, in certain applications such as detecting cracks in long distance fluid transmission pipelines a minimum practical velocity of the sensing device precludes use of a Hall device alone. For example, in the Trans-Alaska pipeline system the problem of providing an effective crack detector on a pig or a device which is transmitted through the line is apparent. The roughly 10.sup.10 square inches of pipe wall provides 10.sup.12 different positions within which a 0.10 by 0.10 inch crack can hide. The signal noise problem is equally as severe. Benign pipe wall anomalies, vibrations of the detector device, changing surface conditions inside and outside, pipe welds and other anomalies create noise in a crack detection device which may produce multiple false alarms. However, the problems associated with providing means for detecting minute changes in magnetic fields such as would be provided by magnetic crack detector are solved by the unique circuit and device of the present invention.